Diesel engines generate nitrogen oxides emissions, which include nitrogen oxide (“NO”) and nitrogen dioxide (“NO2)”, known collectively as “NOx.” To comply with stringent government mandates regarding NOx emissions, engine manufacturers have developed several NOx reduction approaches. One such approach is exhaust gas recirculation (“EGR”), in which a percentage of the exhaust gas is drawn or forced back into the intake and mixed with the fresh intake gas and fuel that enters the combustion chamber. Another approach is selective catalytic reduction (“SCR”). The SCR process reduces NOx to diatomic nitrogen (“N2”) and water (“H2O”), using a catalyst and anhydrous ammonia (“NH3”) or aqueous NH3, or a precursor that is convertible to NH3, such as urea.
In addition to NOx emissions, diesel engines also produce particulate matter (“PM”), or soot. PM is a complex emission that includes elemental carbon, heavy hydrocarbons derived from the fuel, lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. One approach for reducing or removing PM in diesel exhaust is a diesel particle filter (“DPF”). The DPF is designed to collect PM, while simultaneously allowing exhaust gases to pass therethrough.
A diesel oxidation catalyst (“DOC”) may be positioned upstream of the DPF. Among other things, the DOC oxidizes hydrocarbons (“HC”) and converts NO to NO2. Organic constituents that are trapped in the DPF, such as carbon, are oxidized therein, using the NO2 generated by the DOC, so as to form CO2 and H2O, both of which exit into the atmosphere.
Proper operation of the DOC, DPF, and SCR catalyst, which are core components of what is referred to as an aftertreatment system, require operating conditions that are within important temperature parameters.
For example, one temperature parameter is the DOC's “light off” temperature. When below the light off temperature, the DOC's energy level is too low to oxidize HC. The light off temperature is typically around 200-250° C.
Another temperature parameter is related to the conversion of NO to NO2. This NO conversion temperature spans a range of temperatures having both lower and upper bounds, which are defined as the minimum and maximum temperatures at which 40% or greater NO conversion is achieved. The conversion temperature range defined by those two bounds and extends from approximately 200-250° C. to approximately 450° C.
Yet another temperature parameter is related to DPF regeneration. Regeneration involves the presence of conditions that will burn off trapped particulates whose unchecked accumulation would damage the DPF. There are two main forms of regeneration: passive and active.
Passive regeneration is a regeneration that can occur anytime that the engine is operating under conditions that burn off PM without initiating a specific regeneration strategy embodied by algorithms in an electronic control system. Passive regeneration occurs when the DOC inlet temperature is greater than 200-250° C., and conversion becomes greater at higher temperatures with more NO2.
In contrast, active regeneration is a regeneration that is initiated and maintained intentionally by, for example, an electronic control system. The active regeneration raises the temperature of the exhaust gases entering the DPF to a range suitable for initiating and maintaining burning of trapped particulates. The creation of conditions for initiating and continuing active regeneration involves elevating the temperature of exhaust gas entering the DPF to a suitably high temperature.
And still another temperature parameter is related to sulfur removal processes. The presence of sulfur decreases the efficiency of various components in the exhaust aftertreatment system, including the SCR catalyst. Presently known sulfur removal processes require exposing the SCR catalyst to very high temperatures.
There are significant challenges associated with working within these various temperature parameters, particularly when the engine is initially started or operating at low to medium loads. In some aftertreatment systems and during some operating conditions, HC is injected into the exhaust stream, sometimes via a fuel dosing injector positioned upstream of the DOC, or via in-cylinder dosing. While these injections can be useful for raising temperatures in the aftertreatment system, they cannot be used at lower temperatures, as a result of HC having a relatively high oxidation temperature (e.g., 200-250° C.) before it can generate heat via an exothermic reaction in the DOC.